In hold over mode the clock generator remembers the frequency onto which it synchronized. In the event of a failure of all possible reference sources the clock generator can then operate with the last set frequency in synchronous mode.

The standards do not require this function for simple clock generators. In the event of failure of all reference sources, the clock generator can switch to its own average value which can be several ppm off.

A real hold over mode has been implemented for even the simplest, smallest OpenScape 4000 clock generators. With increasing quality this function works even more accurately with standard and networking clock generators. Not only the frequency at the moment of failure is considered (which can be off) but an average value of the last 30 minutes and the last 24 hours is calculated which is then slowly passed by the system in a smooth changeover. With networking clock generators this hold over frequency is stored in a non-volatile memory to prevent its loss during power failure or change of boards. Also, the frequency accuracy during hold over mode relies especially on the temperature dependency of the quartz oscillator used. For standard clock generators, temperature compensated oscillators (TCVCXO) are used with a frequency deviation of only +/- 1 ppb over the entire temperature range. For the background reference of the networking clock generators, at least one temperature stabilized oscillator (OCXO) is used which a frequency deviation of only +/- 1 ppb over the entire temperature range.